Go to main menu -> Where to -> Tracks and select the track you wish to use.

You can also load KMZ files by placing the files in the 'Garmin/KMZ' memory directory.

Garmin GPSmap 60 and eTrex (Using Garmin MapSource)

From the menu, choose File -> Open. Select "GPS eXchange
Format" from the "Files of Type" menu in the dialog. Select the GPX file
you'd like to load.

After loading the GPS track and waypoint data you'd like to transfer, choose Transfer -> Send to Device from the menu.

Select your device and check the "Track" and "Waypoint" checkboxes.

From the main menu, select "Tracks." Select the track you wish
to follow, then select the "TracBack" button. More detailed
instructions can be found here.

Many third party GPS software applications allow you to
transfer track data to and from most Garmin devices. For specific
instructions on transferring GPX data, please consult the software user
manual.

DeLorme PN-series

Using DeLorme's Topo USA software, select the "Draw" tab, then
click the "Import" button. Set the file type input to Files of Type:
GPX.

Select the track file from the list. Click "More." Right click
on the track and choose "Manage Draw" then "Send to GPS." Use the
control or shift key to select multiple tracks.

Click the send button.

You can also select a track directly on the map. Right click on the track and choose "Manage Draw," then "Send to GPS."